How did the instructor determine that its a left-tailed test? Is it because of the negative value of the observed statistic?

Hi @Musab_Bin_Gulfam great question. If you see the hypothesis above it said that null hypothesis is there are no difference between the population mean of A and population mean of B, in other words mA - mB = 0 The alternative hypothesis said that mean of A is lower than mean of B, in other words mA - mB < 0. The hypothesis are defined at the beginning of the exercise and then you conduct the hypothesis testing calculation.

Let me know if this answer your question

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Thank you very much.

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